Polycom VVXTM 1500 - Manual (Page 109)

Customizing Your Phone
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The following table shows the Line Key icons and status indicators that
display for shared lines.
In the following figure, line 1952 is shared and line 1953 is private.
Line Key Icon Shared Line Status Line Key Status Indicator
Idle (No indicator)
Active call Solid red
Call or calls on hold Flashing red